The manufacture of precision parts requires measurement methods for detecting the geometry and condition of the parts to ensure the quality of such parts. Optical measurement methods, for example imaging and image evaluation, interferometry, and white light interferometry play an important role in this process.
The principle of the white light interferometer is based on the fact that a short-coherent light source is used for illuminating an imaging system. In addition to the normal imaging optical system, the imaging system also has a reference arm through which a portion of the incident light passes. If the propagation path of light Λ0 in the object arm differs from the propagation path in reference arm ΛR by an amount which is less than coherence length lc of the light, i.e.,|ΛR−Λ0|<lc  (1)the recombined light fields may have a measurable interference. This is utilized by varying the path difference of the light fields in a defined manner during the measurement and simultaneously measuring the intensity of the recombined light fields on a planarly measuring detector, usually a CCD camera. The pixel-by-pixel evaluation of the intensity modulation generated by the interference, the intensity correlogram, supplies unambiguous height information for each individual pixel. When carried out for the entire pixel field, this results in complete height information for the object.
Commercial white light interferometers typically have the following specifications:
Height resolution Δz is specified by the average wavelength of light μm, coherence length lc and the type of correlogram evaluation algorithm used. Typical parameters such as λm=600 nm, lc=2 μm provide values of Δz=1 nm.
Lateral resolution δ resembles that of a conventional imaging system and, in principle, is limited by λm and numerical aperture NA of the imaging optical system.δ≧0.61ζm/NA  (2)
Maximum measurable total height difference zmax is determined from the technical feasibility of producing a path difference in the reference arm and object arm which is precisely guided over the entire distance. Regulated piezoelectric systems currently support values of zmax≦400 μm.
Conventional interferometers, in particular white light interferometer systems, may be used for the tasks described above if the location to be measured is easily accessible and has a largely flat geometry. If this is not the case, interferometers are used which have a special-purpose optical system which is adjusted to the object to be measured.
For example, interferometers having endoscopic optical systems are known (Lindner MW 2002 “White Light Interferometry Via an Endoscope” Proc SPIE 4777: 90-101) which enable hard-to-reach locations to be measured. The optical endoscope is characterized by the fact that the imaging lenses may be placed so close to the object that numerical aperture NA in equation (2) assumes values which support an adequate resolution δ.
German Published Patent Application No. 101 15 524 describes a white light interferometer in which an optical system for generating a flat intermediate image is situated in the measuring light path. However, the object in this case is to provide a depth scan.
German Published Patent Application No. 100 47 495 describes a white light interferometer having an intermediate image which uses an endoscope for measurements in deep, narrow holes and has a depth resolution of several nanometers.
A measurement system in which the reference arm accommodates an electrically controllable filter for adjusting the intensities of the reference and measuring beams is known from German Published Patent Application No. 101 62 180.
Panoramic optical systems are an important type of special-purpose optical system for white light interferometers, which are adjusted to the component. They provide images of objects positioned at a large angle relative to the optical axis.
However, these optical systems have the disadvantage that the architecture of the interferometric system is determined by the special-purpose optical system to the extent that the system may not be used for any other measuring task using a different special-purpose optical system.